Conventionally, there is known an actuator using a motor for rotationally driving an object member by rotating the motor. Further, as such actuator, there is also known a rod-type actuator adopting a direct-drive type structure in which a ball screw-nut of a ball screw assembly is adopted as such object member and a screw shaft screw-engaged with the ball screw-nut assembly is moved in an axial direction thereof.